Atari and Commodore versions on SAME side of a floppy

[ldelsarte]

Hello to all,
In a video on the YouTube channel of "8-bit show and tell"

Glitch World In Commodore 64 Ninja - YouTube

I discovered a floppy disk version of Ninja (Mastertronics) where the Atari and Commodore versions are on the same side of the disk. Floppy disks with Atari on one side and Commodore on the other side are very common. But I don't understand at all how to fit two versions for two very different computers on the same side of a floppy disk... Can someone explain to me how this is possible? I don't know anything about the Commodore floppy disk format, but I doubt that it is organized like the Atari one. Apart from the fact that they share the physical format 5"1/4 and 48 TPI, the rest must be very different, right? Or is it a common formatting that can be read on both? I'm really curious to know the answer.
Thanks in advance,

 

Edited May 7, 2021 by ldelsarte
tags

[carlsson]

I believe the idea is to format the outer tracks with Atari format, and the inner tracks with Commodore format. The 1541 is starting from track 18 where the directory and BAM (block availability map) are held. From there it will navigate onto other tracks to read the files. If you make sure to store data only on tracks 18-35, the 1541 will never steer into tracks 1-17. Likewise I don't know how Atari disks work, if those start from track 0/1 and then move inwards? The idea has crossed my mind several times, and I believe I've read about such disks before though they probably are less common.

 

Another aspect of course is capacity. A 1541 disk can hold 166K with its GCR encoding and varying number of sectors per track. From what I understand Atari disks depend on DOS version and start from 90K and upwards? But yes if the program is not very large, half the capacity might be enough.

[+Nezgar]

There was an Antic Interview Podcast with the fellow that came up with that method. A great manufacturing efficiency improvement noted the inventor... anyone recall which interview?

 

Ataris boot from track 0 (outermost track) and commodores don't "boot" from a disk, but start from a directory read from the middle of the disk like @carlsson notes so those files can be directed to the inner tracks. Atari DOS also uses middle tracks for directories/VTOC but a game doesnt use DOS so the binary loader can start at track 0 and load from consecutive tracks up to the middle. And since the commodore 1541 (normally) also can't use tracks 36-40, Atari data could be put there too if needed...

[carlsson]

While no Ataris were involved in the making, Trixter brought up a dual IBM/C64 floppy where one side contains both formats and the other side continues the IBM format.

https://trixter.oldskool.org/2008/09/28/the-diskette-that-blew-trixters-mind/

 

In comparison, on the 16-bit systems it almost appears "common" to have disks that would dual boot on Atari ST and Amiga, sometimes also operational on IBM PC as well. The principle must be the same regarding custom formatting.

Edited May 7, 2021 by carlsson

[Rybags]

An Atari bootable floppy only needs the first track to be present (or in fact just that first sector).

Of course logically you'd have much more than just one track as 18 sectors is less than 3K.

 

In theory a Commodore floppy should get by with a minimum of the track/s containing the disk directory and sector map.

Most C= floppies don't use the high track numbers >= 35 (?) so they could be dedicated for the Atari.

 

Also, the presence of the other system's data on known tracks could serve as a sort of simple copy protection - just attempt a read and expect an error.  Though generally C= copy programs are sophisticated enough that they'd copy such a disk with no better protection present, it would be non-trivial to create an Atari disk with readable tracks and the beginning and end but non readable ones in the middle.

[zzip]

Interesting.  I don't remember this being the case, but just pulled out my copy of Ninja and it's true.

 

It's a shame this tecnique wasn't used more often because there was always something disheartening about the Atari version always being put on the back side as if we were lesser.   I suppose some games used too much disk space to make this possible.

[carlsson]

Except for the seam and the index hole, there doesn't need to be a front and a back side. Fortunately neither system utilized the index hole, otherwise that system would have destined to have its version on the same side (and no flippies even if the system was single sided).

[+x=usr(1536)]

1 hour ago, zzip said:

It's a shame this tecnique wasn't used more often because there was always something disheartening about the Atari version always being put on the back side as if we were lesser.   I suppose some games used too much disk space to make this possible.

Best guess is that this was to reduce duplication costs - double-sided duplication would have been more expensive than single-sided, so if two versions of the game could be written to one side of the disk, the manufacturing (and thus overall) cost could be reduced.

[ggn]

ST/Amiga dual format games use pretty much the same principles, but I haven't come across a detailed analysis of how this works. Wikipedia has a list of games that supported dual (and sometimes triple) format.

[Fred_M]

I have Action Biker by Mastertronic on disk and has both versions on 1 side too. Mastertronic marketed these as "Double Play".

 

This is what @Farb found out:

 

"It appears there were two different Mastertronic release of this title - yours is a flippy version with Atari on side B and Fred’s is a (I assume newer) “Double Play” version with both C64 and Atari on the same side. The actual program data is identical between the two but your dump is blank starting at track 16 and Fred’s has missing sectors starting at track 16. So I assume that the “Double Play” release used that area of the disk for the C64 data. Pretty cool"

 

Double Play version, both systems on one side:

 

 

Disk version, on different sides:

 

 

Flippy disk, different sides:

 

Double Play, on same side

 

 

Edited May 7, 2021 by Fred_M

[+Nezgar]

9 hours ago, carlsson said:

disks that would dual boot on Atari ST and Amiga

Interesting.... TIL Amiga also stores it's directory in the middle tracks of a disk.... (track 0 for Atari ST) - Are Amiga's boot sectors there too?

[carlsson]

I read about it here: https://eab.abime.net/showthread.php?t=35332

 

Supposedly the Atari ST/Amiga disks would have interleaved data GCR and MFM on track 0, with carefully placed sync marks and checksums so both computers were able to read parts of track 0 and get the info they needed. If rest of the disk formats some tracks as GCR and other tracks as MFM, I'm not sure.

[ijor]

28 minutes ago, carlsson said:

Supposedly the Atari ST/Amiga disks would have interleaved data GCR and MFM on track 0 ...

Amiga doesn't use GCR. It uses, at least by default, MFM same as the Atari ST. But they use slightly different syncing so that is possible to have separate boot blocks for each platform.

 

[+CharlieChaplin]

you may also take a look at this (old) topic:

 

Commodore and Atari flippie disks

 

One of the inventors of the multi-format disks replied there in post #33.

 

[Rybags]

The Amiga can read/write PC disks with software or later OS, so a game for it + ST could have common code which only needs one copy present.

[carlsson]

Hm.. I could've sworn that the Amiga used a form of GCR but after reading up on it, apparently the hardware supports it but rarely used. That probably made dual booting floppy disks easier. Whether you can load a small portion of CrossDOS or similar software in order to read rest of the disk as FAT formatted to reuse parts of the storage space for multiple formats is interesting. In theory if you had a 1571, it can be coaxed to read MFM, but I don't know if it can decode any A8 disks for a similar case of data sharing between C128 and 130XE. Of course the 1541 can't, at least not without hardware modification so it probably would not be a major use case.

 

Assuming there were custom machines used for disk duplication rather than regular floppy drives connected to some computer, wouldn't those duplicate both sides at once so fitting everything on one disk side or on two sides might not make much of a difference? Sure if the publisher cheaped out and used single sided disks (which often could be used on the back side anyway, but no guarantees), they probably wanted multiple formats on the same disk. Then again most commercial games were quite pricy compared to that double sided disks were common and more or less the norm, so it would have been a really stingy move to increase their margins on using cheaper media.

[+Nezgar]

2 hours ago, carlsson said:

Hm.. I could've sworn that the Amiga used a form of GCR but after reading up on it, apparently the hardware supports it but rarely used.

The Amiga format, even though it is MFM, has no inter-sector gaps. Any time a write is needed, the entire track needs to be erased and rewritten at once which also allows 11 sectors to fit. I can imagine this causing data-loss surprises as a bad or marginal sectors on a track might amplify if they cannot be read out successfully before having to rewrite the track!?

 

As @ijor wrote, it makes sense they could have possibly formatted "half" of track 0 for each platforms boot sectors...

[thorfdbg]

4 hours ago, Nezgar said:

The Amiga format, even though it is MFM, has no inter-sector gaps. Any time a write is needed, the entire track needs to be erased and rewritten at once which also allows 11 sectors to fit.

This is because the Amiga does not have a floppy controller, it is all encoding and decoding by software and the custom chips. Rewriting the entire track is usually not a problem.

 

The Amiga does have a boot sector in track #0 for booting, thus bootable disks that boot off the  boot sector without a file system cannot be shared with the Atari ST. The latter uses FAT as file system and hence uses track 0 for the FAT and the MBR.

 

However, the Amiga file system (OFS/FFS) has its directory in the middle of the disk at sector 880, and this does not interfere with FAT. Thus, in principle, one could create "dual-play" ST/Amiga disks as well, provided the Amiga version uses a custom "startup-sequence" type of boot procedure.

 

[Mclaneinc]

Wow, the joy of AA, always giving info you didn't know...

 

We all knew of flippy's but on the same side, new to me....Never saw one of those in the wild, great idea...

[youxia]

[Mclaneinc]

Good old Rob Northern, better known for his disk protection techniques..

[ggn]

4 hours ago, thorfdbg said:

The Amiga does have a boot sector in track #0 for booting, thus bootable disks that boot off the  boot sector without a file system cannot be shared with the Atari ST. The latter uses FAT as file system and hence uses track 0 for the FAT and the MBR.

I'm not exactly sure what you mean there, but according to the docs (and from my sparse knowledge of the Amiga) there is a bootblock area on the Amiga that's 1024 bytes long. Same goes for the ST, the first sector contains disk info but can also contain executable boot code. File systems are irrelevant during boot, this is read before the file system touches the disk (at least on ST, but I suspect the Amiga too).

 

Unless you meant something else here?

 

4 hours ago, thorfdbg said:

However, the Amiga file system (OFS/FFS) has its directory in the middle of the disk at sector 880, and this does not interfere with FAT. Thus, in principle, one could create "dual-play" ST/Amiga disks as well, provided the Amiga version uses a custom "startup-sequence" type of boot procedure.

You assume that one will be using the file system at all to read and write data. The much easier thing would be to format one side of the disk (barring the first track of side 0) as ST and the other as Amiga. Then both bootsectors can contain code that will just read raw sectors off the disk and thus boot the game (or anything else) in question.

 

Alternatively, if you want to use a file system you could do similar tricks to what trixter described in the linked article, i.e. mark some sectors as "bad" in one file system so they won't be used, and inverse that for the other file system. Since (as you said) the file system areas for both machines reside on different parts of the disk, it's not that impossible.

 

And a small bit of trivia: When we did our version of Lethal Xcess for the Atari ST/E we noted that a large area of both disks wasn't being read at all, so we simply discarded these areas, so we were able to fit the game into a single disk instead of two (which is pretty much what every other group did).

[Rybags]

Generally Amiga bootable games didn't use a filing system, as per Atari 8-bit and many ST games.

 

That cover disk I suspect would be files only... and generally ST cover disks only used an AUTO folder.

 

The thing I'm interested in though is whether you could have coexistent boot sectors for both ST and Amiga.  I suspect the answer is yes.  And I suspect that such a track could probably be created on the Amiga (and ST uses 512 byte boot sector - though that said, DBasic for the ST used a 1K sector custom disk format, with 512 byte boot sector - though I've never seen a disk image of it suitable for emulation)

[carlsson]

I wonder if the fact that the Amiga reads (and writes) an entire track at a time is the reason why Chinon drives in Amiga 500 usually are completely worn out and doubtful if those can be refurbished, while equally old Chinon drives in Atari ST often are perfectly fine.

[ivop]

2 hours ago, Rybags said:

The thing I'm interested in though is whether you could have coexistent boot sectors for both ST and Amiga.  I suspect the answer is yes. 

Interesting. As they both run a 68k, I suppose it should be possible to detect if one is running on an ST or Amiga. Do Amiga disks use the same FAT12 boot sector? Otherwise, it could become tricky.

Edited May 8, 2021 by ivop
typo